Fuel cell stacks are especially envisioned as a source of energy for future mass produced automotive vehicles. A fuel cell stack is an electrochemical device that converts chemical energy directly into electrical energy. A fuel cell stack comprises a stack of a number of cells in series. Each cell generates a voltage of about 1 volt, and stacking them allows a supply voltage of a higher level, for example of about one hundred volts, to be generated.
Among known types of fuel cell stacks, proton exchange membrane (PEM) fuel cell stacks may in particular be mentioned. Such fuel cell stacks have particularly advantageous compactness properties. Each cell comprises an electrolytic membrane that only lets protons pass, the passage of electrons being blocked. The membrane comprises an anode on a first side and a cathode on a second side.
At the anode, dihydrogen, used as fuel, is ionized to produce protons that pass through the membrane. The electrons produced by this reaction migrate toward a flow plate then flow through an electrical circuit external to the cell so as to form an electrical current. At the cathode, oxygen is reduced and reacts with the protons to form water.
The fuel cell stack may comprise a number of flow plates, for example made of metal, stacked on one another. The membrane electrode assembly (MEA) is placed between two flow plates. The flow plates may comprise channels and orifices for guiding reactants and products to/from the membrane. The plates are also electrically conductive in order to form collectors for collecting the electrons generated at the anode. Gas diffusion layers are interposed between the electrodes and the flow plates and make contact with the flow plates.
The flow plates make contact with very acidic solutions. On the cathode side, the plate is subjected to pressurized air in a highly oxidizing environment. On the anode side, the plate makes contact with hydrogen. Under such conditions, the metal plates corrode. Corrosion of a plate leads, on the one hand, to emission of metal ions that adversely affect the operation of the electrolytic membrane and, on the other hand, to formation of an insulating oxide layer on the metal, thereby increasing its contact resistance with respect to the gas diffusion layer. The electrical resistance between the plate and the gas diffusion layer is thus increased. These effects degrade the performance of the fuel cell stack.
The metal plates must therefore have a high electrical conductivity while not corroding or oxidizing. As most metals are subject to the same problems of oxidation and corrosion, a certain number of studies have proposed to form a protective coating on the metal plates in order to preserve the electrical conduction properties of the metal from which they are made while preventing oxidation reactions occurring on their surface.
In the document entitled “Deposition of gold-titanium and gold-nickel coatings on electropolished 316L stainless steel bipolar plates for proton exchange membrane fuel cells” published in the International Journal of Hydrogen Energy, 2010, Vol. 35, No. 4, pp. 1713-1718, the metal plates were in particular coated with gold. Thus, the contact resistance between a plate and a gas diffusion layer was reduced. However, the cost of gold and its deposition process make such a solution impractical for mass production.
Document US 2008/248368 describes a fuel cell stack equipped with a separating collecting plate covered with a hydrophilic layer including a polyurethane matrix. A gas diffusion layer is covered with a hydrophobic layer made of polytetrafluoroethylene.
U.S. Pat. No. 7,267,869 proposes to form a protective layer on metal plates by depositing a polymer-based ink containing a carbon particle filler. The purpose of the carbon particles is to improve conduction between the gas diffusion layer and the metal plate.